The present invention relates to a method of and an apparatus for feeding a bar to an automatic lathe. More particularly this invention concerns a feed apparatus for a screw machine with a sliding headstock.
In a screw machine a bar is fed to the machining location through a hollow headstock between machining operations. In addition, with sliding headstock machines, the headstock moves axially, that is parallel to the elongation direction of the bar which also corresponds to the turning axis of the machine. Such an arrangement is in principle described in the commonly assigned patent application Ser. No. 594,285 filed July 9, 1975, now U.S. Pat. No. 4,006,654 and the references cited therein.
In such a feed apparatus typically a pusher rod is provided at its front end, that is its end turned toward the headstock, with a chuck, by which is meant any workpiece holder, whether of the collet, clamping-sleeve, or any other type, that holds the rear end of the bar rod. A chain drive connected via a releasable clutch to a motor has a chain connected to the pusher rod so as to axially advance the bar. A magazine is provided for loading in fresh bars when necessary and a two-part openable guide is provided for guidance of the bar even during rotation and for guidance of the pusher rod behind the bar.
In such devices the pusher rod typically pushes at all times on the rear end of the bar. Such constant pushing has the effect with small-diameter bars of occasionally bending them laterally and requiring the apparatus to be shut down and the bent bar removed. Furthermore such pushing frequently deflects the bar laterally out of the guide so as to make further feeding impossible. It should also be noted in this context that when very small clearance, often no more than 5 mm, is provided the chuck which is provided with a bearing so that it can connect the rotating bar with the nonrotating pushing rod, wears rapidly since in combination to this small-diameter bearing the rotational speed is extremely high to achieve optimal cutting speed.
It has been suggested to improve on the above-described type of drive by using a nonsynchronous motor having relatively low torque and considerable slip. The use of an electronicaly controlled direct-current motor has also been suggested in this regard. Both systems have the considerable disadvantage that they exert uneven pressure on the rear end of the bar. Furthermore, such systems rarely are able to exert sufficient force to remove the tail end of a bar from the headstock after a machining operation when the remaining piece is insufficiently long to form another workpiece. Thus it is essential in such installations that a separate extracting arrangement be provided, thereby increasing equipment investment.
A relatively sure system has been designed which uses a sleeve that grips the bar immediately behind the headstock. Such an arrangement, although relatively sure in operation, is very complicated and, therefore, greatly increases the cost of the automatic lathe. Furthermore any device which is rotationally coupled to the bar behind the headstock must also be rotated by the headstock so that the torsion load on the headstock is increased considerably. Thus any type of arrangement which increases the mass of the bar behind the headstock will lead to premature wearing out of this headstock.
Various prior-art suggestions can be seen in Swiss Pat. No. 551,828, German published specifications Nos. 1,402,279 and 2,362,130 as well as in German published application No. 1,117,968.